1. Field of the Invention
The present invention relates to a terminal demodulation apparatus and method of a communication system. More particularly, the present invention relates in a terminal demodulation apparatus and method in an Orthogonal Frequency Division Multiplexing System.
2. Description of the Related Art
Generally, an Orthogonal Frequency Division Multiplexing (OFDM) scheme used in wireless communication systems is a modulation scheme having a plurality of carriers. The OFDM systems are well known to have a superior modulation/demodulation performance under a mobile receive environment that is subject to multi-path interferences and fading effect.
FIG. 1 illustrates a downlink frame of an OFDM signal.
A preamble symbol 100 of the frame is used to acquire a synchronization of time, acquire a synchronization of frequency, search a cell in which a terminal is included, and estimate a channel.
Mobile Allocation Part (MAP) information 110 of the frame includes demodulation-related information and information related to the state of a base station. Particularly, the MAP information 110 includes location information of a user data burst, size information of the user data burst, information related to modulation scheme, and information related to subcarrier allocation method.
Accordingly, when the terminal does not correctly demodulate the MAP information 110, the terminal cannot receive other information included in the frame. The MAP information 110 often has a variable length according to the number of the terminals provided service by the present base station.
Various types of user data 120, 130, and 140 are allocated. The user data 120, 130, and 140 are allocated in various subcarrier allocation schemes and modulation schemes according to the channel state of to the respective terminals. For example, when the terminal has a good channel environment and requires data of a large capacity, the base station modulates the data in 64 QAM (Quardrature Amplitude Modulation) and allocates data in ⅚ channel coding. However, when the terminal has a bad channel environment, the base station allocates the data in QPSK (Quadrature Phase Shift Keying) modulation and in 1/12 channel coding, and if not satisfied, may allocate data in a subcarrier allocation scheme having a lot of pilot carriers in a symbol.
For example, in FIG. 1, the user data 120 is modulated in the subcarrier allocation Type 1 and in 16 QAM, the user data 130 is modulated in the subcarrier allocation Type 2 and in QPSK, and the user data 140 may be modulated in the subcarrier allocation scheme Type 2 and in 64 QAM.
FIG. 2 illustrates various subcarrier allocation schemes capable of being selected according to a channel environment of between the base station and terminal.
The subcarrier allocation scheme Type 1 may be used for the terminal having a good channel state, in which the number of the pilot subcarriers is given as 1 for 8 data subcarriers. The subcarrier allocation scheme Type 2 may be used for the terminal having a bad channel state, in which a ratio of the pilot subcarriers with respect to the data subcarrier is given as 1:2. The more pilot subcarriers are given, the more accurately the channel state is estimated. Accordingly, the information damaged in the bad channel may be correctly recovered.
Although it is efficient to use various modulation schemes and subcarrier allocation schemes according to the terminal-channel state, the structure is difficult and power consumption is increased because the terminal must support all of the types of modulation schemes and subcarrier allocation schemes.
FIG. 3 is a block diagram of a conventional OFDM demodulation apparatus.
The conventional OFDM demodulation apparatus includes an analog-digital converter (ADC) 300, a fast Fourier transform (FFT) unit 310, a rear-FFT memory 320, a scramble code generator unit 330, a channel estimator 340, an equalizer 350, a QAM demapper 360, and a channel decoder 370.
The ADC 300 receives input analog signals, transforms the received analog signals into digital signals, and outputs the transformed signals.
The FFT unit 310 performs a FFT on the digital signals received from the analog-digital converter 300, that is, the data subcarriers and pilot subcarriers and transforms the digital signals into data symbols of the frequency domain.
The rear-FFT memory 320 stores data output from the FFT unit 310, and reads the data. The scramble code generator unit 330 generates pseudo-noise scramble codes and output the generated codes.
The scramble code generator 330 includes a scramble code generator 332 for generating and outputting scramble codes and a scramble code memory 334 for storing the scramble codes output from the scramble code generator 332.
The scramble code generator unit 330 is preloaded with many scramble codes, and outputs the corresponding scramble code according to a subcarrier allocation algorithm.
The multiplying unit 380 performs a descrambling function by multiplying the extracted subcarriers of the memory 320 and the scramble code so as to decrease an inter-cell interference.
The channel estimator 340 detects a pilot subcarrier from among the subcarriers read from the memory 320, compares the detected pilot subcarrier to the reference pilot signal, estimates a channel according to changed phases, and transmits the channel estimate to the equalizer 350.
The equalizer 350 detects a data subcarrier among the subcarriers read from the memory 320, equalizes the detected data subcarrier using the channel estimate received from the channel estimator 340, and accordingly, eliminates channel interference.
The QAM demapper 360 demodulates an output value of the equalizer and transmits the demodulated data to the channel decoder 370. The channel decoder 370 decodes the transmitted data.
Such a demodulation method is not appropriate for a frame having various modulation methods and subcarrier allocation methods.
Generally, since a bit size of the received signal of the terminal is determined according to a bit size of the ADC 300, data size output from the analog-digital converter 300, that is, the bit size of the received signal of the terminal, is relatively small.
However, the bit size of the received signal of the terminal increases when it passes the FFT unit 310.
Accordingly, the rear-FFT memory 320 provided after the FFT unit 310 needs more memory capacity than a memory (not shown) provided in the front of the FFT unit 310.
The subcarrier allocation scheme or modulation scheme-related information will be known when the demodulation of the map information of in the front of the frame and the decoding of the channel decoder 370 is finished. Accordingly, the rear-FFT memory 320 stores a great amount of symbol data, and accordingly, the rear-FFT memory 320 has a large load.
The conventional demodulation apparatus will have a decreased hardware load by descrambling in which the subcarrier and scramble code extracted from the rear-FFT memory 320 are sequentially multiplied. This is because the output of the FFT unit 310 may be sequentially transmitted through a re-ordering and may be directly multiplied by the scramble code generated by the scramble code generator 330.
However, in some systems such as the portable Internet system, the scramble code may randomly change in the middle of a frame, information about whether a new code starts from any a symbol are described in the MAP information 110.
For example, in FIG. 1, the user data 102 may use a scramble code Type 1, and the user data 103 and the user data 104 may use a scramble code Type 2.
Accordingly, the conventional demodulation apparatus shown in FIG. 3 must store the output values of the FFT unit 310 in the not-descrambled state until the map decoding is finished, and then multiply the scramble code when the subcarrier values are read from the rear-FFT memory 320.
In addition, a process for reading the subcarrier from the rear-FFT memory 320 is non-sequentially performed according to a complex subcarrier allocation algorithm. Accordingly, the hardware load becomes increased because the scramble code generator 330 previously generates a few hundred or a few thousand scramble code samples, stores the generated samples, reads the same according to the subcarrier allocation algorithm, and multiplies the same by the scramble codes.
The rear-FFT memory 320 of the conventional demodulation apparatus shown in FIG. 3 simultaneously stores a pilot subcarrier and a data subcarrier.
The channel estimator 340 reads a pilot subcarrier and the equalizer 350 reads a data subcarrier. Operation for reading the rear-FFT memory 320 must be time-shared so as to simultaneously read the two types of information. That is, the demodulation apparatus must read the rear-FFT memory 320 using a clock running two or more times faster than a system clock, and accordingly, the demodulation apparatus has an increased load.
One attempt to solve such a problem, considered that the pilot subcarrier and the data subcarrier are respectively stored at the separate memories. However, such a method is impossible because the subcarrier allocation method of the received symbol is not known before the map decoding is finished.